1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically to a semiconductor device including a refresh control circuit.
2. Description of Related Art
Semiconductor devices such as a dynamic random access memory (DRAM) need periodic refresh operation for the sake of maintaining the content stored in memory cells. Such a semiconductor device performs auto refresh operation when an auto refresh command (REF command) is supplied to the semiconductor device. The semiconductor device performs self-refresh operation when a self-refresh command (SELF command) is input (Japanese Patent Application Laid-Open Nos. H5-2872 and H6-275071).
When a REF command is supplied, a semiconductor device typically activates certain word lines in all memory banks constituting the semiconductor device with predetermined periods of delay between the memory banks. Specifically, when a REF command is supplied, the semiconductor device converts the REF command into refresh activation signals REFACT for controlling the activation of word lines and the activation of sense amplifiers in the respective memory banks. The refresh activation signals REFACT are supplied to the memory banks with time differences therebetween. All the memory banks perform a refresh operation (referred to as a first refresh control operation) based on the respective input refresh activation signals REFACT.
When a SELF command is supplied, the semiconductor device converts the SELF command into refresh activation signals REFACT and performs the first refresh control operation. When a SELF command is supplied, the semiconductor device also makes an internal oscillator produce self-excited oscillations. The oscillator periodically generates a refresh control signal. Based on the refresh control signals generated by the oscillator, the semiconductor device outputs the refresh activation signals REFACT to all the memory banks with time differences as in the first refresh control operation. All the memory banks each activate certain word lines in the memory bank and perform a refresh operation as with the first refresh control operation. In other words, when a SELF command is supplied, the semiconductor device initially performs the first refresh control operation like when a REF command is supplied. The semiconductor device then repeats the first refresh control operations based on the refresh control signal which the oscillator periodically generates.
The foregoing semiconductor device can reduce peaks of the power supply current supplied to the entire semiconductor device by activating certain word lines in all the memory banks with predetermined periods of delay between the respective memory banks. However, there has been a problem that the peak value of the power supply current in self-refresh operation is the same as in auto refresh operation since the first refresh control operation is repeated even in self refresh operation.
To terminate self refresh operation, a self refresh exit command (SELF Exit command) needs to be supplied to the semiconductor device. During self refresh operation, the semiconductor device performs the first refresh control operation based on the refresh control signal periodically generated by the oscillator as described above. When a SELF Exit command is supplied to the semiconductor device in time with the generation of the refresh control signal by the oscillator, then the semiconductor device completes the first refresh control operation before ending self refresh operation. As mentioned above, the first refresh control operation refreshes all the memory banks with delays. The termination of self refresh operation therefore lags as much as the delay times that are set to shift the start times of refresh operations between the memory banks. There has thus been a problem that the time (exit time from self refresh) between when a SELF Exit command is supplied to the semiconductor device and when the semiconductor device ends self refresh operation and it becomes possible to accept a next command is long.